1. Field of the Invention
This invention relates to the fabrication of multiple glass/glass, glass/plastic, and plastic/plastic laminates for ballistic resistant and security applications.
2. Technology Review
Glass laminates as related to the ballistic and security applications involve the construction of alternate layers of glass lites and plastic sheets with an adhesive interlayer. For the most part the interlayer has been a meltable film based on polyvinyl butyral. This polymer film has been the basis of substantially all safety glass applications in automobile windshields and commercial/residential windows requiring impact resistance. In some areas, however, problems exist that made the use of polyvinyl butyral film difficult at best.
Originally, bullet resistant glass consisted primarily of alternating layers of glass lites ranging in thickness from 1/8 to 3/8 and polyvinyl butyral film, that was 30 to 90 mils in thickness. Since the film contains 30% plastizer, the film rolls must be kept cool prior to use. The film is cut to the desired shape and placed between two lites of glass. The glass/film/glass composite is run through an oven in order to melt the film. Entrained air in the film is removed at this point by either vacuum or by roll compression. About 95% of the air is removed at this stage. The laminate is now subjected to 240-300F. temperature and 60-200 lb/sq. in. pressure in an autoclave to dissolve residual air, maintain film thickness and to achieve acceptable adhesion to glass. The production of multilayers of glass lites held together with polyvinyl butyral film is difficult at best.
With the introduction of tough, new plastics such as polycarbonate and acrylic sheets to replace some of the glass lites in order to produce light weight bullet resistant laminates, the problem associated with the use of polyvinyl butyral (P.V.B.) in multilayer applications became more severe. Adhesion of P.V.B. to the polycarbonate surface is marginal at best and delamination with time can occur. The high temperatures and pressures of the autoclave also can deteriorate the optical properties of the polycarbonate sheet. Alternative film development based on polyurethane has been commercialized and is currently offered as a replacement for P.V.B. in bullet resistant glass clad/polycarbonate composites. Although the adhesion to the polycarbonate has improved and impact strength is superior, air entrainment and increased costs vs. P.V.B. are disadvantages. Since the autoclave is still required, visual distorting of polycarbonates still can occur. Alternate urethane chemistry involves the application of a liquid, curable interlayer. In one case, the mix is comprised of two components-- an isocyanate prepolymer and a polyol crosslinker. Advantages of this system is high temperatures and high pressures treatment are not necessary.
Adhesion to the polycarbonate sheet is acceptable but the disadvantage is the removal of air when the liquid mix is introduced between the glass and polycarbonate sheets. Nominal thickness for bullet resistant glass clad polycarbonate sheets is 20-40 mils. A liquid mix with a viscosity greater than 300 cps. at 75.degree. F. is extremely difficult to introduce in this space and at the same time remove entrained air. Other versions of urethane/acrylate modifications or methacrylate base liquid adhesives either etch the polycarbonate sheets or have severe air inhibition cure problems.
Unsaturated polyester resin systems have been used as liquid adhesives especially for television tube glass laminates. These types of resins can be formulated to have acceptable impact and energy absorbing properties for bullet resistant applications, but the major monomer employed in these types systems is styrene and styrene severely etches the surface of polycarbonate sheet, thus destroying the optical properties.
There have been prior patents regarding methods of making laminated glass using a liquid thermosetting resin. Two United States patents U.S. Pat. No. 3,703,425 to Delmonte and U.S. Pat. No. 4,234,533 to Langlands describe procedures for fabricating glass laminates using liquid thermosetting adhesives.
In the process described in the Delmonte patent, a flat glass lite is bowed in the center with a suction cup. Porous, double sided tape is applied at the edges of the glass. A second piece of glass is laid on top of the first. The double sided tape then seals the two pieces of glass together with a pool of resin between the lites in the center. The function of the tape is resin containment, thickness control and an outlet for entrapped air. A vacuum is applied around the glass lite. Air is removed through the tape pulling the resin to the tape. The glass laminate now remains flat until the liquid adhesive polymerizes to a solid.
In the process described in the Langlands patent, two lites of glass are adhered together with double sided porous tape at the edge on three sides. The fourth side is taped only on the edge of the lower plate. The protective film is partially removed. The two glass lites are fixed at a 45.degree. angle. The resin is now introduced into a space between the lites. With 1/8 glass lites, the glass bows to accommodate the resin. As the glass composite is lowered to a flat position the resin fills the void with the entrained air escaping at the fourth side. Removal of the protective film allows the laminate to be completely sealed. Any remaining trapped air can be removed by applying pressure to the surface of the laminate. The entrained air either escapes through the porous tape or is removed with a needle syringe.
It is an object of this invention to provide an improved simple yet effective procedure for fabrication of glass clad/polycarbonate and acrylic bullet resistant laminates.